In order to reduce a driving operation of a vehicle performed by an operator thereof, a vehicle drive control device for executing an automatic drive control, such as a cruise control for executing a constant speed control by which the vehicle speed is controlled to be a target vehicle speed, an adaptive cruise control (ACC) for controlling the speed of the vehicle to follow a vehicle traveling in front of the subject vehicle (hereinafter referred to as a leading vehicle) while maintaining a distance between the subject vehicle and the leading vehicle, and the like, is adapted to the vehicle. In such a vehicle drive control device, a target driving force is calculated by an automatic drive control ECU as a target control variable so that the vehicle speed is controlled to be a target vehicle speed. Then, the calculated target driving force is outputted to an engine ECU. The engine ECU operates an engine, which serves as a vehicle speed adjusting apparatus for adjusting the vehicle speed on the basis of the target driving force. In such a condition, the vehicle speed is detected by detecting rotational speed of wheels of the vehicle by means of a wheel speed sensor and is controlled to be the target vehicle speed. Further, in the known driving apparatus for the vehicle, the automatic drive control is canceled when an operator performs a braking operation.
Recently, there exists a need for performing an automatic drive control at low speed, for example, at 10 kilometers per hour. In the known driving apparatus for the vehicle, when the operator performs the braking operation while the vehicle drives on a sloped road at the low speed due to the automatic drive control, the automatic drive control may be canceled. When the automatic drive control is canceled while the vehicle drives on the sloped road at the low speed and in a condition where the vehicle is not allowed to stop at the sloped road by the braking force generated by the braking operation of the operator, a position of the vehicle may not be maintained. Therefore, for example, the vehicle may move downwardly, thus a behavior of the vehicle may change.
Because of the above described circumstances, a vehicle drive control device, which does not cancel the automatic drive control even when the operator performs the braking control, has been proposed, For example, JP2004-90679A (hereinafter, referred to as reference 1) discloses a drive control device, by which an automatic drive control ECU reduces a target speed and calculates a target driving force for changing the vehicle speed to the reduced target speed and the engine ECU operates an engine on the basis of the target driving force calculated by the automatic drive control ECU without canceling the automatic drive control even when the operator performs the braking operation, thus lowering the vehicle speed.
When the vehicle drives on a curved road, the speed difference is generated between inner wheels and outer wheels. However, the vehicle includes a differential mechanism, which is a driving force transmitting apparatus for absorbing the speed difference between the inner wheels and outer wheels and further for applying the same level of driving torque from a power source (engine) to the inner wheels and outer wheels. Accordingly, in a condition where one of plural wheels of the vehicle slips while the automatic drive control is performed because of contacting a road surface of which frictional coefficient μ is low, a large driving force may be transmitted to the slipping wheel(s) while driving force transmitted to wheels which are not slipping may be reduced. Therefore, according to the vehicle drive control device of the reference 1, in a condition where one of the wheels slips while the automatic drive control is performed, the braking force is applied to the slipping wheel(s) by a brake apparatus so as to restrain the slipping of the wheels. Thus, by restraining the slipping of the vehicle, the driving force of the engine is transmitted to the other wheels, which are not slipping and applied with less braking force. Therefore, the vehicle is kept driving by the driving force. Though even in such a state where the slipping of the wheel(s) is restrained by the braking force of the brake apparatus, in a condition where the braking force is reduced because of fading phenomenon of a brake pad, for example, the slipping of the wheels may not be restrained.
FIG. 4 is an explanatory view illustrating a wheel speed, a brake torque and all engine output in a condition where one of the wheels slips, according to a known drive control device. Herein, a vehicle speed 353 obtained when the automatic drive control is executed is calculated on the basis of the rotational speed of the wheels detected by the wheel speed sensor. However, in a condition where the rotational speed of one of the wheels is high because of the slipping, the vehicle speed 353 is calculated on the basis of the rotational speed of other wheels which are not slipping and of which rotational speed is low. Thus, in a condition where one of the wheels is slipping, a slipping wheel speed 351, which is a rotational speed of the slipping wheel, becomes high while the vehicle speed 353, which is calculated as a speed of the vehicle being driven, is calculated as a low speed because of being calculated on the basis of a non-slipping wheel speed 352 which is the rotational speed of a non-slipping wheel.
In such a state, because the vehicle drive control device controls the rotational speed of the slipping wheel to be reduced by means of the brake apparatus, a slipping wheel brake torque 355 which corresponds to the brake torque applied to the slipping wheel becomes larger than a non-slipping wheel brake torque 356 which corresponds to the brake torque applied to the non-slipping wheel. However, even in a case where the slipping wheel brake torque 355 is controlled to be larger than the non-slipping wheel brake torque 356, the slipping wheel speed 351 is not reduced when the braking force is reduced due to the fading phenomenon of the brake pad. Therefore, the driving force of the engine is transmitted to the non-slipping wheel(s). Therefore, the non-slipping wheel speed 352 is kept to be low, and the vehicle speed 353 is accordingly calculated to be low on the basis of the non-slipping wheel speed 352.
In such a state, the vehicle speed 353 does not reach a target vehicle speed 365 at the condition where the automatic drive control is performed. Therefore, the vehicle drive control device controls an engine request output 360, which corresponds to level of the output requested to the engine, to be increased so that the vehicle speed 353 approaches the target vehicle speed 365. However, when the slipping wheel keeps slipping, the driving force of the engine is transmitted to the slipping wheel even though the engine output is increased. Accordingly, after a fading occurrence time 371, the slip wheel speed 351 increases while the non-slipping wheel speed 352 is kept to be low.
Accordingly, the vehicle speed 353 calculated on the basis of the non-slipping wheel speed 352 is kept to be low, while the vehicle drive control device controls the engine request output 360 to be further greater so as to match the vehicle speed 353 to the target vehicle speed 365. Thus, in a state where the calculated vehicle speed 353 is controlled to approach (match) the target vehicle speed 365 while one of the wheels is slipping, the output of the engine may be excessively increased, thus causing a malfunction of the vehicle, by damaging the engine and/or a drive train, for example.
A need thus exists for a vehicle drive control device which is not susceptible to the drawback mentioned above.